Hydro-Point: Ultrasound sign of air-fluid level in the pleura

Introduction with a case

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Hydro-point on thoracic ultrasound is diagnostic of an intrapleural air-fluid level, which may be caused by hydropneumothorax, hemopneumothorax, or pyopneumothorax. This has been described in the literature but is not widely appreciated (Volpicelli 2013). For example, a prominent ultrasound website may have misdiagnosed this. Before going further, watch the video and see what you think:

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Lung point: Diagnostic of pneumothorax

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Before understanding hydro-point, it is useful to briefly review lung point (a sign which is diagnostic of pneumothorax). Lung point occurs when the ultrasound probe is placed at the junction between a pneumothorax and lung (image below). As the patient breathes in and out, the ultrasound image alternates between the image of pneumothorax (A-lines without lung slide) and normal lung slide.

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Below is a video by Neurocritical Care Ultrasound illustrating lung point. If this isn't making sense, see the review of pneumothorax diagnosis by rebelem.com.

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Hydro-point: Diagnostic of pleural air-fluid level

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Hydro-point was first introduced by Targetta 1992 to describe the ultrasound finding of a hydropneumothorax. However, this finding may be seen with any air-fluid level in the pleura (including hemopneumothorax or pyopneumothorax). This sign is seen when the ultrasound probe is at the level of the air-fluid interface (figure below). The air-fluid interface oscillates up and down due to wavelets in the fluid caused by respiratory or cardiac motion. This causes the image to alternate between showing a pneumothorax and a pleural effusion.

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Below is a film clip of hydro-point obtained from a patient with a hemopneumothorax at Genius General Hospital. This predominantly shows a hemothorax with underlying lung consolidation, with the ultrasound beam frequently interrupted by interposed air from the pneumothorax. This effusion was on the patient's left side near his heart, so the movement of the hydro-point seems to reflect a syncopated rhythm combining his heart rate and respiratory rate.

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Sorry about the image quality. This is a schematic of what is seen on the video:

Below is a schematic of what is seen on this video. This video is zoomed in more than most, so the entire pleural effusion cannot be seen:0

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To make a secure diagnosis of hydropneumothorax, the following three components should be seen. These points correspond to the figure:

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[A]Superior to the air-fluid level, the ultrasound should show pneumothorax.

[B]At the air-fluid level, the ultrasound should show hydro-point (oscillation between an effusion and a pneumothorax).[C]Inferior to the air-fluid level, the ultrasound should show a pleural effusion.

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The performance of hydro-point seems very good although data is limited. Combining series by Targetta 1992 and Lin 2004, it has a sensitivity of 29/30 (97%). I'm unaware of any reports of false-positives.

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Differential diagnosis of hydro-point

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(1) Lung point may be confused with hydro-point as both represent rather abrupt interfaces which include a pneumothorax. These may be differentiated because the hydro-point alternates between pneumothorax and pleural effusion, whereas the lung point alternates between pneumothorax and lung tissue.

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(2) A lung abscess with an air-fluid interface near the chest wall can mimic hydro-point. Lin 2004 compared 16 patients with lung abscess to 19 patients with pyopneumothorax, all of whom had peripheral air-fluid levels on chest radiograph or CT scan. They found both hydro-point and loss of lung slide above this location (i.e., [A] and [B] above) in 18/19 patients with pyopneumothorax and no patients with lung abscess. Although air-fluid levels were observed within 10/16 of the lung abscesses on ultrasound, these air-fluid levels did not extend to the pleura (figure below).0

Lung abscess creating an air-fluid level (white arrowhead). This may be differentiated from hydro-point because it does not contact the pleura. From Lin 2004

Hydro-point may be due to hydropneumothorax, hemopneumothorax, or pyopneumothorax (i.e., empyema containing a gas-fluid level). Attention to the fluid characteristics may provide some clues regarding underlying etiology. For example, hemothorax may be suggested by the appearance of clotting blood within the fluid, whereas micro-bubbles suspended in the fluid suggest pyohydrothorax:0

Lin 2001 found suspended micro-bubbles in 8/8 cases of pyopneumothorax and no cases of uninfected hydropneumothorax. However, a later study by Chen 2009 found microbubles in only 23% of pyopneumothorax cases. Image from Lin 2001.

Recently Volpicelli 2014 showed that the location of lung point on the chest correlates with the size of a pneumothorax (figure below). In a supine patient, a small pneumothorax will rise to the anterior portion of the chest, and therefore the lung point will be located anteriorly. As the pneumothorax grows in size, the lung point moves to a progressively more posterior location. In the case of complete lung collapse, the lung point may disappear entirely. The location of hydro-point may have similar implications regarding the underlying size of the pneumothorax versus the pleural effusion in a supine patient. For example, a hydro-point located on the anterior chest would suggest a small pneumothorax with a very large effusion. Alternatively, hydro-point located on the posterior thorax would suggest a smaller pleural effusion and a more significant pneumothorax. For patients with pleural adhesions or loculations (i.e., a patient in the ICU who has had multiple chest tubes), this may not apply.0

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Take-home points

Hydro-point is the ultrasonographic representation of an air-fluid level in the pleural space. The ultrasound displays rhythmic alternation between showing a pneumothorax and showing a pleural effusion.

The combination of hydro-point, a pneumothorax anterior to the hydro-point, and a pleural effusion posterior to the hydro-point allows fairly confident diagnosis of an intra-pleural air-fluid interface. This may be caused by hydropneumothorax, hemopneumothorax, or pyopneumothorax.

Hydro-point should be differentiated from lung point, which reveals alternation between a pneumothorax and normal lung slide and is diagnostic of a pneumothorax.